1. Field of the Invention
The invention relates to a sheet-fed scanning device capable of detecting a document edge, and more particularly to a sheet-fed scanning device utilizing a dark background which does not contact the document to facilitate the detection of the document edge.
2. Description of the Related Art
In a sheet-fed scanner, the conventional document edge detecting method includes detecting a leading edge of a document in order to inform the scanner when to start outputting the acquired image signal of the document. A method for detecting the leading edge of the document may be performed by utilizing a paper sensor in conjunction with the design parameters of the sheet-feeding structure. This method, however, may cause corresponding mechanical errors. Another method for detecting the leading edge of the document may be performed by utilizing a dark background behind the document. In this detection method, the document edge is assumed to be white, so the dark background may serve as a comparison reference. The leading edge detection may be achieved by way of image processing.
After the document leading edge has been detected, it is possible to judge whether or not the segments at two ends of the leading edge of the document reach the scanning region simultaneously by way of image processing, and the skew correction of the document may be performed accordingly.
FIG. 1 is a schematic illustration showing a conventional device for detecting a leading edge of a document. As shown in FIG. 1, a document 102 moving across a scan window 105 is scanned by a scanning module 104 under the scan window 105. A background component 101 is positioned above the document 102 and in contact with the document 102 to thereby provide a black background to facilitate the detection of a leading edge 103 of the document and the subsequent skew correction.
Under the long-term usage, the background component 101 may be correspondingly worn out and then lose its smooth surface. In result, the fed document may be scratched and the scan quality may be influenced owing to the impact of the increasing frictional force between the document and the background component 101 on the speed of the document moving across the scanning region. In addition, the background component 101 also may attract a lot of paper flakes due to the electrostatic charges. If the paper flakes are white, the background component 101 cannot provide the black background and so loses its function. Thus, the detection of the document leading edge and the skew correction may be further influenced.
Consequently, it is an important subject of the invention to provide a sheet-fed scanning device, which is capable of detecting a document edge, suitable for the long-term usage, and free from the worn-out problem.